Nucleotide sequences from the third open reading frame or mRNA D(D3) of infectious bronchitis virus (IBV) were expressed in bacteria as part of a fusion protein with ฮฒ-galactosidase. Antiserum raised in rabbits against this fusion protein immunoprecipitated from IBV-infected chick kidney or Vero cells a polypeptide of 12.4 K, the size expected for a D3-encoded product. The D3 polypeptide is apparently non-glycosylated, and appears to be associated with the membrane fraction of infected cells, as judged by cell fractionation and immunofluorescence.